The polemicist implicitly conceded both source series were mutually uncertain

Evidence

Earlier in the same work he had written that all those things which were alleged by Christian, and seemed to be referred to by Muslim sources, represented ‘a strong presumption, even certain proof’ of the Christian assertions. This statement is too strong; it does not really represent his attitude. In practice he did not treat either series as certainly true; he treated them as being to an unknown extent mutually confirmatory. — Norman Daniel, Islam and the West, 1314